Considering the fact that Bitcoin market prices are now mature enough to be streaming on a Bloomberg terminal doesn't it make sense to create a cash market price driven fee model so this never has to be revisited in the future?
Empowering each node with this insight would also assist future directions for the GUI where people invariably will want to send/receive/report in fiat terms. If the fiat price is already used by all nodes to facilitate transaction fee handling, an additional getfiatpx call could be added to the RPC interface.